1. Field of the Invention
This invention relates to a transfer sheet. More particularly, it relates to a transfer sheet which is a transfer material to which, in electrophotographic apparatus or electrostatic printers, a toner image obtained by forming an electrostatic latent image on an image-bearing member such as a photosensitive member and developing the electrostatic latent image is transferred, and an image-forming method making use of such a transfer sheet.
2. Related Background Art
In electrophotographic apparatus, after an electrostatic latent image has been formed on a photosensitive member, the toner of a developer is made to adhere electrostatically to the electrostatic latent image to form a toner image, and this toner image is transferred to a transfer sheet (paper) by means of a transfer assembly. As transfer assemblies of this type, electrostatic transfer means such as corona transfer means and roller transfer means are known in the art.
The progress of electrophotography has taken place in copying machines. With the spread of its application to output machinery, such as page printers and facsimile machines, it has made an advance from analog systems to digital systems and is increasingly demanded to achieve higher function, more coloration and higher image quality.
Nowadays, in most electrophotographic apparatuses, the toner image held on the photosensitive member is transferred to plain paper by an electrostatic transfer means as mentioned above, where images may greatly deteriorate at the time of transfer. This deterioration causes inferior images formed by printing and ink-jet recording.
Recently, in the field of ink-jet recording other than electrophotography, it was really shocking that replacement of sheets with special exclusive sheets has brought about a dramatic improvement in image quality.
In respect of sheets of transfer sheets for electrophotography, too, a variety of proposals have been made in order to improve transfer performance and image quality. In particular, properties having energetically been studied include electrical properties, such as volume resistivity and surface resistivity of sheets. For example, in Japanese Patent Publications No. 41-20152 and No. 43-4151, it has been proposed to maintain volume resistivity within a stated range; in Japanese Patent Application Laid-Open No. 50-117435, it has been taught to provide a resin layer having a volume resistivity of 3xc3x971013 xcexa9xc2x7cm or above on the surface of transfer paper; and it has been taught in Japanese Patent Application Laid-open No. 56-16143, to provide on a transfer paper""s base layer firstly a low-resistance layer and then at the outermost surface a high-resistance layer to make up a transfer sheet. In an actual service environment, however, it has been so difficult to control moisture in the air and that it has been unable to stabilize electrical resistance of transfer sheets. Accordingly, as disclosed in Japanese Patent Application Laid-open No. 5-53363, it is proposed to incorporate in a sheet a synthetic hectorite having a specific crystal structure, attempting to make a resistance value environmentally stable. Even this proposal, however, cannot provide images on the level comparable to the level of those formed by ink-jet recording or by printing.
As an approach from a different aspect, there has been a method in which an elastomer is coated on the surface of transfer paper, as disclosed in Japanese Patent Application Laid-open No. 49-126334. In an attempt to make image evaluation on a color electrophotographic apparatus by actually coating on transfer paper the material disclosed therein, no remarkable effect was observable with regard to the reproduction of a photographic image on a 400 dpi digital printer.
As the cause of image deterioration in the transfer process as stated above, it can be concluded that, a dithered pattern formed as a result of image processing employed by recent printers or a toner image formed of continuous minute individual dots by PWM (pulse width modulation) stands scattered when digital data are outputted. This tends more remarkably in the case of, e.g., very fine dots of a screen on which small characters or image data are formed. A one-dot toner image that constitutes binary image data of 400 dpi has a size of about 64 xcexcm. As for the improvement in dot reproducibility of about such size, it cannot be expected at all by any conventional means stated above, showing capability not different at all from ordinary transfer sheets. More specifically, in conventional means, ink-jet recording enables reproduction of 800 dpi photographic images, whereas electrophotographic processing has been unsatisfactory in any effort to reproduce true 400 dpi photographic images, because of the image deterioration (a decrease in gradation) caused in the transfer process.
However, even though the means disclosed in the above Japanese Patent Application Laid-open No. 49-126334 is old, the inventors have been interested in that its means relies on a mechanical phenomenon which may hardly be affected by environmental factors, different from other resistance values or the like. However, has been found that, even for soft elastomers used at present in, e.g., intermediate transfer members of the latest color copying machines, it is difficult to transfer binary images (toner images) of 400 dpi without scattering.
Japanese Patent Application Laid-open No. 9-170190 discloses a transfer sheet made to have a fibrous surface as a recording sheet for output machinery of various types. This publication discloses that its fibers exhibit a cushioning performance and hence can make dry-process electrophotographic toner images sharp. However, as also shown in its Examples, the thickness of the fiber used, though fairly as small as 0.5 denier, is only on the level of the particle size of electrophotographic toners. Hence, the cushioning performance exhibited by fibers which mutually slide as so described in the above publication may be expectable for making large-size characters or the like sharp at best, but is not so expectable as to absorb kinetic energy of individual toner particles as aimed in the present invention. Materials disclosed as examples in the above publication are celluloses and polyester resins, which are materials of the same nature as, or harder than, those of toners, and hence, as the materials alone, they are not expectable at all for any cushioning performance on individual toner particles.
An object of the present invention is to provide a transfer sheet having solved the above-noted problems, and an image-forming method making use of the transfer sheet.
More specifically, an object of the present invention is to provide an electrophotographic transfer sheet which has a superior effect of keeping dot toner images from scattering at the time of transfer, and an image-forming method making use of the transfer sheet.
To achieve the above-noted object, the present invention provides a transfer sheet comprising a base layer and a surface layer formed on at least one surface of the base layer, wherein;
in a plot graph with load P (mN) as ordinate and the square of indentation depth A (xcexcm) as abscissa, plotted when the tip of a diamond triangular pyramid penetrator having a dihedral angle of 80xc2x0 is pressed in on the side of the surface layer;
the plot graph has a first flexing point that appears first, a first region extending from the first flexing point to zero and a second-and-further region subsequent to the first flexing point; and
a gradient H of the graph in the first region is 0.09 mN/xcexcm2 or smaller; and
the base layer includes paper made from pulp.
The present invention also provides an image-forming method comprising;
a toner image forming step of forming a toner image by means of a toner; and
a transfer step of transferring the toner image formed, to a transfer sheet;
wherein;
the transfer sheet has a base layer and a surface layer formed on at least one surface of the base layer; and
in a plot graph with load P (mN) as ordinate and the square of indentation depth A (xcexcm) as abscissa, plotted when the tip of a diamond triangular pyramid penetrator having a dihedral angle of 80xc2x0 is pressed in on the side of the surface layer;
the plot graph has a first flexing point that appears first, a first region extending from the first flexing point to zero and a second-and-further region subsequent to the first flexing point; and
a gradient H of the graph in the first region is 0.09 mN/xcexcm2 or smaller. and
the base layer includes paper made from pulp.